1. Field of the Invention
This invention relates to a method and an apparatus for measuring the frequency response of, and more particularly, to a magnetoelectric method and apparatus for measuring the droplet frequency response of an ink jet printhead.
2. Description of Related Art
For most commercial inkjet printers, printing graphics and documents is normally carried out by the printhead. In principle, a printhead of an inkjet printer heats up the ink and vaporizes the ink to form ink bubbles by converting electric energy into heat. The printhead then jets the ink drops, which are developed from the ink bubbles, onto a destination surface through spouts. In order to speed up the printing efficiency of an inkjet printer, the manufacturers normally focus on increasing the droplet frequency response. That is, the droplet frequency response indicates the printing speed of an inkjet printer. Hence, how to measure the droplet frequency response of an inkjet printhead has become a very important technique in inkjet printer manufacture.
The droplet frequency response is obtained by comparing the detected actual jetting frequency of an inkjet printhead with the driving frequency actually applied to the inkjet printhead. The maximum droplet frequency response of the inkjet printhead can be measured by checking the matching between different driving frequencies applied on the inkjet printhead and the actual responding jetting frequencies of the inkjet printhead. Since the ink bubbles are generated at the printhead in a frequency varied from several kilo-Hertz (kHz) to several tens kHz, it is impossible to detect the actual droplet frequency response through a regular image mapping system. Even though utilizing a high-speed camera it is possible to catch the actual droplet frequency response of an inkjet printhead, and determine the droplet frequency response of the inkjet printhead, it is not cost effective. Hence, some apparatuses and methods have been developed for the purpose of measuring droplet frequency response of an inkjet printhead, such as those disclosed by U.S. Pat. Nos. 4,484,199 and 4,590,482.
The schematic cross-sectional diagram of a conventional measuring apparatus for determining the droplet frequency response is illustrated in FIG. 1.
Referring to FIG. 1, a planar detecting electrode 106 is placed parallel to a metallic nozzle plate 100, and a voltage difference exists between the detecting electrode 106 and the nozzle plate 100. The detecting electrode 106 and the nozzle plate 100 are not electrically connected, though the distance between them is quite short, for example less than 100 .mu.m. Once an ink drop 104 is jetted by the nozzle plate 100 through nozzle 102, the ink drop forms an electric connection between the detecting electrode 106 and the nozzle plate 100 before the ink drop 14 totally leaves the nozzle plate 100. The electric connections formed by continuously jetted ink drops out of the nozzle plate 100 can be detected by an attached electronic circuit (not shown in figure) for obtaining the forming frequency of the ink drops. However, ink drops are easily stuck within the narrow space between the detecting electrode 106 and the nozzle plate 100, and that leads to an error reading on the forming frequency of ink drops while a detecting process is performed.
The schematic cross-sectional diagram of another conventional measuring apparatus for determining the droplet frequency response is illustrated in FIG. 2.
Referring to FIG. 2, a pair of electrodes 208 is placed between the nozzle plate 200 and the detecting electrode 206, wherein a high voltage is applied on the electrodes 208 to provide a high-voltage electric field. While an ink drop 204 jetted by the nozzle plate 100 passes through the electrodes 208, the ink drop is charged. An electric signal can then be detected at the detecting electrode 206 after the charged ink drop hits the detecting electrode 206. By counting the number of the electric signals within a period of time, the forming frequency of the ink drops is obtained. An ink drop, which is about 100 pico liters (pl) in volume, is possibly broken into several sub-drops while the ink drop 204 passes through the high-voltage electric field says, exceeding 1000 volts. Therefore, the detected forming frequency at the detecting electrode is interfered by the noise signals given by the sub-drops.